Point-of-use ammonia purification for electronic component manufacture

ABSTRACT

Highly purified ammonia for use in processes for the production of high-precision electronic components is prepared on-site by drawing ammonia vapor from a liquid ammonia reservoir, passing the vapor through a filter capable of filtering out particles of less than 0.005 micron in size, and scrubbing the filtered vapor in a high-pH aqueous scrubber.

This invention lies in the field of the manufacture of high-precisionelectronic components, and relates to the preparation and handling ofthe ammonia used as a treatment agent in the manufacture of suchcomponents.

BACKGROUND OF THE INVENTION

A major concern at every stage in the manufacture of electroniccomponents is contamination. Control of contamination is critical toproduct quality, and an extremely high level of cleanliness and purityin the manufacturing environment is needed for obtaining acceptableproduct yield and maintaining profitability. These requirements areparticularly acute in the manufacture of very high density circuitry aswell as in ultra-precision bearings, recording heads and LCD displays.

Sources of contamination include the manufacturing facility, personneland processing equipment. In many cases, contamination can be lowered toacceptable levels by the use of "clean room" techniques such asisolation, air filtration, special equipment and special clothing andbody coverings to avoid contact between the operator and themanufacturing materials. With ultra-high precision manufacturing,however, the highest levels at which defects can be tolerated areparticularly low and control over sources of contamination is even morecritical.

Ammonia presents particular difficulties, since liquid ammonia containsboth solid and volatile impurities, many of which are damaging toelectronic components if present during the manufacturing process. Theimpurities level and content may vary widely depending on the source aswell as the handling method, and all such impurities must be removedbefore the ammonia can be used in electronic component production lines.

To meet this standard, production facilities have had to obtainhigh-quality ammonia at considerable cost from the limited sources whichare able to supply ammonia at an acceptable grade. Only qualifiedsuppliers can be used, and new suppliers must be qualified before theirproduct can be accepted. This cost and the lack of flexibility addconsiderably to the cost of the components.

Further difficulties arise in meeting Department of Transportationregulations. Ammonium hydroxide are shipped at concentrations no higherthan 30%.

Clearly there is a need for a reliable means of supplying ammonia at apurity level which will produce a high yield of acceptable product inultra-high precision components, and which can meet the requirements ofadvancing electronics technology.

SUMMARY OF THE INVENTION

It has now been discovered that ammonia can be supplied to a productionline for high-precision electronic devices in ultra-high purity form byuse of an on-site system which draws ammonia vapor from a liquid ammoniareservoir, passes the ammonia vapor through a microfiltration filter,and scrubs the filtered vapor with high-pH purified water. Theuniqueness of this discovery is that it can convert commercial gradeammonia to ammonia of sufficiently high purity for high-precisionmanufacturing without the need for conventional column distillation. Thedrawing of the ammonia vapor from the supply reservoir serves by itselfas a single-stage distillation, eliminating non-volatiles orhigh-boiling impurities, such as alkali and alkaline earth metal oxides,carbonates and hydrides, transition metal halides and hydrides, andhigh-boiling hydrocarbons and halocarbons. The reactive volatileimpurities that could be found in commercial grade ammonia, such ascertain transition metal halides, Group III metal hydrides and halides,certain Group IV hydrides and halides, and halogens, previously thoughtto require distillation for removal, are now discovered to be capable ofremoval by scrubbing to a degree which is adequate for high-precisionoperations. This is a highly unusual discovery, since scrubbertechnology is traditionally used for the removal of macro-scale, ratherthan micro-scale, impurities. In the present invention, the scrubberlowers the levels of impurities which are damaging to semiconductorwafer manufacture to less than 1 ppb per element or less than 30 ppbtotal. For operations where even greater purity is desired, distillationmay also be performed subsequent to the scrubbing. An advantage of theinvention, however, is that if distillation is included, the scrubberconsiderably lessens the burden on, and design requirements for, thedistillation column, enhancing the product purity even further. Theremoval of impurities which are close-boiling relative to ammonia, suchas reactive hydrides, fluorides and chlorides, simplifies thedistillation column design considerably. While this system and processare applicable to ammonia utilization sites at high-precision productionlines in general, the invention is of particular interest for thepurification of ammonia used at semiconductor wafer cleaning stations.

These and other features, embodiments, applications and advantages ofthe invention will be apparent from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an engineering flow diagram of one example of a unit for theproduction of ultrapure ammonia in accordance with the presentinvention.

FIG. 2 is a block diagram of a semiconductor fabrication line in whichthe ammonia purification of FIG. 1 may be incorporated, thereby servingas one example of an implementation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with this invention, ammonia vapor is first drawn from thevapor space in a liquid ammonia supply reservoir. Drawing vapor in thismanner serves as a single-stage distillation, leaving certain solid andhigh-boiling impurities behind in the liquid phase. The supply reservoircan be any conventional supply tank or other reservoir suitable forcontaining ammonia, and the ammonia can be in anhydrous form or anaqueous solution. The reservoir can be maintained at atmosphericpressure or at a pressure above atmospheric if desired to enhance theflow of the ammonia through the system. The reservoir is preferably heatcontrolled, so that the temperature is within the range of from about10° C. to about 50° C., preferably from about 15° C. to about 35° C.,and most preferably from about 20° C. to about 25° C.

Impurities that will be removed as a result of drawing the ammonia fromthe vapor phase include metals of Groups I and II of the Periodic Table,as well as aminated forms of these metals which form as a result of thecontact with ammonia. Also included will be oxides and carbonates ofthese metals, as well as hydrides such as beryllium hydride andmagnesium hydride. Further included will be Group III elements and theiroxides, as well as ammonium adducts of hydrides and halides of theseelements. Still further are transition metal hydrides. Heavyhydrocarbons and halocarbons such as pump oil will aim be included.

The ammonia drawn from the reservoir is passed through a filtration unitto remove any solid matter entrained with the vapor. Microfiltration andultrafiltration units and membranes are commercially available and canbe used. The grade and type of filter will be selected according toneed. Preferred filters are those which eliminate particles of 0.005micron or greater in size, and further preferred are those which filterdown to 0.003 micron particle size.

The filtered vapor is then passed to a scrubber in which the vapor isscrubbed with high-pH purified (preferably deionized) water. The high-pHwater is preferably an aqueous ammonia solution, the concentrationraised to saturation by recycling through the scrubber. The scrubber maybe conveniently operated as a conventional scrubbing column incountercurrent fashion. Although the operating temperature is notcritical, the column is preferably run at a temperature ranging fromabout 10° C. to about 50° C., preferably from about 15° C. to about 35°C. Likewise, the operating pressure is not critical, although preferredoperation will be at a pressure of from about atmospheric pressure toabout 30 psi above atmospheric. The column will typically contain aconventional column packing to provide for a high degree of contactbetween liquid and gas, and preferably a mist removal section as well.

In one presently preferred example, the column has a packed height ofapproximately 3 feet (0.9 meter) and an internal diameter ofapproximately 7 inches (18 era), to achieve a packing volume of 0.84cubic feet (24 liters), and is operated at a pressure drop of about 0.3inches of water (0.075 kPa) and less than 10% flood, with arecirculation flow of about 2.5 gallons per minute (0.16 liter persecond) nominal or 5 gallons per minute (0.32 liter per second) at 20%flood, with the gas inlet below the packing, and the liquid inlet abovethe packing but below the mist removal section. Preferred packingmaterials for a column of this description are those which have anominal dimension of less than one-eighth of the column diameter. Themist removal section of the column will have a more dense packing, andis otherwise conventional in construction. It should be understood thatall descriptions and dimensions in this paragraph are examples only.Each of the system parameters may be varied.

In typical operation, startup is achieved by first saturating deionizedwater with ammonia to form a solution for use as the starting scrubbingmedium. During operation of the scrubber, a small amount of liquid inthe column sump is drained periodically to remove accumulatedimpurities.

Examples of impurities that will be removed by the scrubber includereactive volatiles such as silane (SiH₄) and arsine (AsH₃), halides andhydrides of phosphorus, arsenic and antimony, transition metal halidesin general, and Group III and Group VI metal halides and hydrides.

The units described up to this point may be operated in eitherbatchwise, continuous or semi-continuous manner. Continuous orsemi-continuous operation is preferred. The volumetric processing rateof the ammonia purification system is not critical and may vary widely.In most operations for which the present invention is contemplated foruse, however, the flow rate of ammonia through the system will be withinthe range of about 200 cc/h to about 2 L/h.

Ammonia leaving the scrubber can be further purified by distillationprior to use, depending on the particular type of manufacturing processfor which the ammonia is being purified. When the ammonia is intendedfor use in chemical vapor deposition, for example, the inclusion of adehydration unit and a distillation unit in the system will bebeneficial. The distillation column may also be operated in eitherbatchwise, continuous or semi-continuous manner. In a batch operation, atypical operating pressure might be 300 pounds per square inch absolute(2,068 kPa), with a batch size of 100 pounds (45.4 kg). The column inthis example has a diameter of 8 inches (20 cm), a height of 72 inches(183 cm), operating at 30% of flood, with a vapor velocity of 0.00221feet per second (0.00067 meter per second), a height equivalent to atheoretical plate of 1.5 inches (3.8 era), and 48 equivalent plates. Theboiler size in this example is about 18 inches (45.7 cm) in diameter and27 inches (68.6 cm) in length, with a reflux ratio of 0.5, andrecirculating chilled water enters at 60° F. (15.6° C.) and leaves at90° F. (32.2° C.). Again, this is merely an example; distillationcolumns varying widely in construction and operational parameters can beused.

Depending on its use, the purified ammonia, either with or without thedistillation step, may be used as a purified gas or as an aqueoussolution, in which case the purified ammonia is dissolved in purified(preferably deionized) water. The proportions and the means of mixingare conventional.

A flow chart depicting one example of an ammonia purification unit inaccordance with this invention is shown in FIG. 1. Liquid ammonia isstored in a reservoir 11. Ammonia vapor 12 is drawn from the vapor spacein the reservoir, is then passed through a shutoff valve 13, thenthrough a filter 14. The filtered ammonia vapor 15, whose flow iscontrolled by a pressure regulator 16, is then directed to a scrubbingcolumn 17 which contains a packed section 18 and a mist removal pad 19.Saturated aqueous ammonia 20 flows downward as the ammonia vapor flowsupward, the liquid being circulated by a circulation pump 21, and theliquid level controlled by a level sensor 22. Waste 23 is drawn offperiodically from the retained liquid in the bottom of the scrubber.Deionized water 24 is supplied to the scrubber 17, with elevatedpressure maintained by a pump 25. The scrubbed ammonia 26 is directed toone of three alternate routes. These are:

(1) A distillation column 27 where the ammonia is purified further. Theresulting distilled ammonia 28 is then directed to the point of use.

(2) A dissolving unit 29 where the ammonia is combined with deionizedwater 30 to form an aqueous solution 31, which is directed to the pointof use.

(3) A transfer line 32 which carries the ammonia in gaseous form to thepoint of use.

The second and third of these alternatives, which do not utilize thedistillation column 27, are suitable for producing ammonia with lessthan 100 parts per trillion of any metallic impurity. For certain uses,however, the inclusion of the distillation column 27 is preferred.Examples are furnace or chemical vapor deposition (CVD) uses of theammonia. If the ammonia is used for CVD, for example, the distillationcolumn would remove non-condensables such as oxygen and nitrogen, thatmight interfere with CVD. In addition, since the ammonia leaving thescrubber 17 is saturated with water, a dehydration unit may beincorporated into the system between the scrubber 17 and thedistillation column 27, as an option, depending on the characteristicsand efficiency of the distillation column.

With any of these alternatives, the resulting stream, be it gaseousammonia or an aqueous solution, may be divided into two or more branchstreams, each directed to a different use station, the purification unitthereby supplying purified ammonia to a number of use stationssimultaneously.

A conventional cleaning line for semiconductor fabrication is depictedin FIG. 2. The first unit in the cleaning line is a resist strippingstation 41 where aqueous hydrogen peroxide 42 and sulfuric acid 43 arecombined and applied to the semiconductor surface to strip off theresist. This is succeeded by a rinse station 44 where deionized water isapplied to rinse off the stripping solution. Immediately downstream ofthe rinse station 44 is a cleaning station 45 where an aqueous solutionof ammonia and hydrogen peroxide are applied. This solution is suppliedin one of two ways. In the first, aqueous ammonia 31 from the dissolvingunit 29 shown in FIG. 1 is combined with aqueous hydrogen peroxide 46,and resulting the mixture 47 is directed to the cleaning station 45. Inthe second, pure gaseous ammonia 32 from the like-numbered line in FIG.1 is bubbled into an aqueous hydrogen peroxide solution 48 to produce asimilar mixture 49, which is likewise directed to the cleaning station45. Once cleaned with the ammonia/hydrogen peroxide combination, thesemiconductor passes to a second rinse station 50 where deionized wateris applied to remove the cleaning solution. The next station is afurther cleaning station 54 where aqueous solutions of hydrochloric acid55 and hydrogen peroxide 56 are combined and applied to thesemiconductor surface for further cleaning. This is followed by a finalrinse station 57 where deionized water is applied to remove the HCl andH₂ O₂, and finally a drying station 58. The wafer or wafer batch 61 willbe held on a wafer support 52, and conveyed from one workstation to thenext by a robot 63 or some other conventional means of achievingsequential treatment. The means of conveyance may be totally automated,partially automated or not automated at all. Note that purified HCl forthe acid cleaning station 54 may be prepared and supplied on site in amanner similar to that of the ammonia purification system of FIG. 1.

The use of ammonia and hydrogen peroxide as a semiconductor cleaningmedium at workstations such as the cleaning station 45 shown in FIG. 2is well known throughout the industry. While the proportions vary, anominal system would consist of deionized water, 29% ammonium hydroxide(weight basis) and 30% hydrogen peroxide (weight basis), combined in avolume ratio of 6:1:1. This cleaning agent is used to remove organicresidues, and, in conjunction with ultrasonic agitation at frequenciesof approximately 1 MHz, removes particles down to the submicron sizerange.

The ammonia purification system will be positioned in close proximity tothe point of use of the ammonia in the production line. The ammonia cantherefore be directly applied to the semiconductor substrate withoutpackaging or transport and without storage other than a small in-linereservoir, and thus without contact with the potential sources ofcontamination normally encountered when chemicals are manufactured andprepared for use at locations external to the manufacturing facility.The distance between the point at which the ammonia leaves thepurification system and its point of use on the production line willgenerally be less than about one foot (30 cm). This distance will begreater when the purification system is a central plant-wide system forpiping to two or more use stations, in which case the distance will beabout twenty feet (6.1 m) or less. Transfer can be achieved through anultra-clean transfer line of a material which does not introducecontamination. In most applications, stainless steel or polymers such ashigh density polyethylene or fluorinated polymers can be usedsuccessfully.

Due to the proximity of the ammonia purification unit to the productionline, the water used in the unit can be purified in accordance withsemiconductor manufacturing standards. These standards are commonly usedin the semiconductor industry and well known among those skilled in theart and experienced in the industry practices and standards. Methods ofpurifying water in accordance with these standards include ion exchangeand reverse osmosis. Ion exchange methods typically include most or allof the following units: chemical treatment such as chlorination to killorganisms; sand filtration for particle removal; activated charcoalfiltration to remove chlorine and traces of organic matter; diatomaceousearth filtration; anion exchange to remove strongly ionized acids; mixedbed polishing, containing both cation and anion exchange resins, toremove further ions; sterilization, involving chlorination orultraviolet light; and filtration through a filter of 0.45 micron orless. Reverse osmosis methods will involve, in place of one or more ofthe units in the ion exchange process, the passage of the water underpressure through a selectively permeable membrane which does not passmany of the dissolved or suspended substances. Typical standards for thepurity of the water resulting from these processes are a resistivity ofat least about 15 megohm-era at 25° C. (typically 18 megohm-cm at 25°C.), less than about 25 ppb of electrolytes, a particulate content ofless than about 150/cm³ and a particle size of less than 0.2 micron, amicroorganism content of less than about 10/cm³, and total organiccarbon of less than 100 ppb.

In the process and system of this invention, a high degree of controlover the product concentration and hence the flow rates is achieved byprecise monitoring and metering using known equipment andinstrumentation. A convenient means of achieving this for ammonia is byvapor pressure measurement. Other methods will be readily apparent tothose skilled in the art.

The foregoing is offered primarily for purposes of illustration. It willbe readily apparent to those skilled in the art that furthermodifications, substitutions and variations of various kinds can be madein terms of the many system parameters discussed above without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A system for the preparation of ultra-high purityammonia, said system comprising:(a) a reservoir of liquid ammonia with avapor space above said liquid ammonia; (b) means for drawing vaporcontaining ammonia gas from said vapor space; (c) a filtration membraneremoving particles greater than 0.005 micron from vapor thus drawn; and(d) an aqueous ammonia scrubber containing an aqueous solution ofammonia in deionized water, arranged to contact filtered vapor havingpassed through said filtration membrane with said aqueous solution, thevapor thus scrubbed being purified ammonia gas.
 2. A system inaccordance with claim 1, further comprising a distillation columnarranged to distill vapor emerging from said scrubber.
 3. A system forthe manufacture of a high-precision electronic component, said systemcomprising:(a) a production line containing a plurality of workstationssuccessively arranged for treating a workpiece to be formed into saidelectronic component, one such workstation selected for application ofammonia to said workpiece; (b) means for conveying said workpiece tosaid workstations in succession along said production line; and (c) asubunit adjoining said production line at said selected workstation tosupply said ammonia in ultra-high purity form, said subunitcomprising:(i) a reservoir of liquid ammonia with a vapor space abovesaid liquid ammonia; (ii) means for drawing vapor containing ammonia gasfrom said vapor space; (iii) a filtration membrane removing particlesgreater than 0.005 micron from vapor thus drawn; and (iv) a scrubberarranged to contact filtered vapor having passed through said filtrationmembrane with an aqueous solution of ammonia in deionized water, thevapor thus scrubbed being purified ammonia gas; and (d) means forapplying the product of step (c) directly to a workpiece at saidworkstation;said production line, said conveying means and said subunitall being free of contamination by semiconductor manufacturing standardsdefined as a resistivity of at least about 15 megohm-cm at 25° C., lessthan about 25 ppb of electrolytes, a particulate content of less thanabout 150/cm³ and a particle size of less than 0.2 micron, amicroorganism content of less than about 10/cm³, and total organiccarbon of less than 100 ppb.
 4. A system in accordance with claim 3 inwhich said subunit further comprises a distillation column arranged todistill vapor emerging from said scrubber.
 5. A system in accordancewith claim 3 in which said subunit further comprises means for combiningsaid purified ammonia gas with purified water to form an aqueous ammoniasolution.
 6. A system in accordance with claim 3 further comprising anammonia outlet on said scrubber, said ammonia outlet positioned withinapproximately 30 cm of said means for applying the product of step (c)directly to said workpiece.
 7. A system in accordance with claim 3 inwhich said subunit is sized to produce said purified ammonia gas at arate of from about 200 cc/h to about 2 L/h.
 8. A system in accordancewith claim 3 in which components (ii), (iii) and (iv) of said subunitare arranged for continuous or semi-continuous flow.
 9. A method forsupplying a high-purity ammonia reagent to a workstation in a productionline for the manufacture of a high-precision electronic component, saidmethod comprising:(a) drawing ammonia gas from a vapor space aboveliquid ammonia in an ammonia-containing reservoir; (b) passing saidammonia gas through a filtration membrane removing particles greaterthan 0.005 micron therefrom; (c) passing said ammonia gas thus filteredthrough a scrubber wherein said ammonia gas is contacted with an aqueoussolution of ammonia in deionized water; and (d) recovering said ammoniagas emerging from said scrubber and directing said ammonia gas to saidworkstation.
 10. A method in accordance with claim 9 further comprisingdissolving said ammonia gas emerging from said scrubber in purifiedwater prior to directing said ammonia gas to said workstation.
 11. Amethod in accordance with claim 9 further comprising passing saidammonia gas through a distillation column for further purification priorto directing said ammonia gas to said workstation.
 12. A method inaccordance with claim 9 further comprising:(c') passing said ammonia gasfrom said scrubber through a distillation column for furtherpurification, and dissolving said ammonia gas emerging from saiddistillation column in purified water prior to directing said ammoniagas to said workstation.
 13. A method in accordance with claim 9 inwhich step (c) is conducted at a temperature ranging from about 10° C.to about 50° C.
 14. A method in accordance with claim 9 in which step(c) is conducted at a temperature ranging from about 15° C. to about 35°C.
 15. A method in accordance with claim 12 in which steps (c) and (c')are conducted at a temperature ranging from about 15° C. to about 35° C.16. A method in accordance with claim 9 in which step (c) is conductedat a temperature ranging from about 15° C. to about 35° C. and at apressure of from about atmospheric pressure to about 30 psi aboveatmospheric pressure.
 17. A method in accordance with claim 12 in whichsteps (c) and (c') are conducted at a temperature ranging from about 15°C. to about 35° C. and at a pressure of from about atmospheric pressureto about 30 psi above atmospheric pressure.
 18. A method in accordancewith claim 9 in which said scrubber is positioned within approximately30 cm of said workstation.
 19. A method in accordance with claim 12 inwhich said distillation column is positioned within approximately 30 cmof said workstation.